Catalytic cracking processes are conventionally employed to produce gasoline and light distillate fractions from heavier hydrocarbon feed stocks. These cracking processes include fixed bed operations and fluid catalytic riser cracking processes. The catalysts employed in such conventional processes for the cracking of hydrocarbons boiling substantially above 600.degree. F. generally contain silica or silica-alumina, such materials frequently being associated with zeolitic materials. The zeolitic materials which are conventionally ion exchanged with suitable cations such as the rare earths can be natural occurring or could have been produced by conventional ion exchange methods so as to provide metallic ions which improve the catalyst activity of the molecular sieve zeolitic-modified silica-alumina catalysts.
Examples of cracking catalysts to which the method of this invention is applicable include those obtained by admixing an inorganic oxide gel with an aluminosilicate and aluminosilicate compositions which are strongly acidic in character as a result of treatment with a fluid medium containing at least one rare earth metal cation and a hydrogen ion or one capable of conversion to a hydrogen ion. Other cracking catalyst compositions which can be employed in conventional processes and the process of this invention include those crystalline aluminosilicate zeolites having a mordenite crystal structure.
Petroleum charge stocks to the catalytic cracking processes contain metals which are generally in an organometallo form, such as in a porphyrin or naphthenate with such metals tending to be deposited in a relatively non-volatile form onto the catalyst. Those metals contained as contaminants in hydrocarbon feeds to the catalytic processes include nickel, vanadium, copper, chromium, and iron. The accumulation of such metal contaminants onto the catalyst may exceed 1500 parts per million nickel equivalents (ppm nickel + 0.2 ppm vanadium).
As an integral step of the hydrocarbon cracking process, the cracking catalyst is regenerated by heating the catalyst to elevated temperatures generally in the range of about 800.degree. to about 1600.degree. F. (427.degree. to 871.degree. C.) for a period of time ranging from 3 to 30 minutes, in the presence of a free oxygen-containing gas. This process step in a separate regenerator is conducted so as to reduce the concentration of the carbon on the catalyst to less than 0.3 weight percent.
Conventional operation of the regeneration step results in the production of carbon dioxide and carbon monoxide which are withdrawn from the regenerator in the effluent gaseous stream. Normally, the ratio of carbon dioxide to carbon monoxide on a volume basis is less than 2.0. If the volume ratio of carbon dioxide to carbon monoxide at a constant excess oxygen and substantially constant regenerator bed temperature could be increased in the regeneration zone, there would result an increase in the heat of combustion from the coke. This increased heat of combustion would increase bed temperature at constant coke make and would result in desirably reduced levels of carbon on regenerated catalyst and improved gasoline and light distillate products yields of the cracking process.
A greater heat release in the regeneration zone from the more complete conversion of the carbon monoxide to carbon dioxide can be an aid in heat balancing the process of cracking low boiling feeds, hydrogenated feeds and paraffinic charge stocks. Such feed stocks have low aromatic carbon contents (C.sub.A) as defined by the following classification method:
C.sub.a = 0.2514 + 0.00065 tw + 0.0086 S - 0.00605 .times. AnPt + 0.00257 (AnPt/Sp.Gr.) where PA0 Tw = Weight average boiling point (.degree. F.) PA0 AnPt = Aniline Point, ASTM D-611, (.degree. F.) PA0 S = weight percent sulfur PA0 Sp.Gr. = Specific gravity (60/60.degree. F.).
when the C.sub.A value is lower than 12 volume percent the coke yield may not be sufficient to provide the heat required to satisfy the reactor heat duty at normal carbon monoxide to carbon dioxide ratio levels.